The Laboratory of Cell Biology (LCB) conducts basic research on notch proteins, primarily human and murine notch-1. Notch proteins are transmembrane receptors which participate in the control of cell fate determination in several tissues and organs, including among others the hematopoietic, immune and nervous systems. Human notch-1 was originally identified because a mutation resulting in irreversible receptor activation causes about 10% of the cases of T-cell acute leukemia. Furthermore, notch-1 is overexpressed in a number of solid tumors, and monoclonal antibodies to its intracellular region are being studied as diagnostic agents in cervical, colon and lung carcinomas. Recently, evidence that notch-1 controls cell fate decisions during T cell development in mice has been published. In adult mammals, notch-1 is expressed in undifferentiated, stem cell compartments in various organs. The only two postmitotic cell types in which it is expressed are mature T cells and neurons. During FY' 97, The LCB has made a number of significant advances in the study of notch-1, which are of direct relevance to its potential biotechnological applications: 1) we further improved the expression system originally developed by LM , which was used to produce a biologically active notch-1-derived protein in soluble form with 6 disulfide bonds. This vector has now been made stable in the complete absence of antibiotic selection (LM, CF); 2) we have shown that notch-1 expression is necessary for the induction of differentiation by experimental anticancer drugs of the hybrid polar family in Friend erythroleukemia cells. This effect of notch-1 is exerted early during exposure to the differentiating agent and is independent of the G1 prolongation which is known to be induced by these drugs (LS); 3) we have developed and are presently characterizing several novel notch-1 monoclonal antibodies with potential clinical applications (CF) and 4) we have initiated and are currently continuing the systematic characterization of the in vivo transgnic model of notch-1 deficiency we generated over the past 2 years (LS, LM, EL). The LCB is now building on these results and using the novel biologics it developed to study: 1) the role of notch-1 in the mechanism of action of other differentiation-inducing chemotherapeutic agents (CF, LS); 2) the function of notch-1 in differentiated neuronal cells in vitro (CF); 3) the level of expression of notch-1 in malignancies other than the ones already characterized, to clarify the possible role of this protein as a tumor marker (CF) and 4) the physiological role of notch-1 in hematopoietic and T cells in the transgenic mouse model we developed (LM, LS, EL).